Spindle motor

ABSTRACT

There is provided a means for preventing wear caused by the contacting/sliding of a thrust hydrodynamic gas bearing surface when a spindle motor is started. A cylinder  4  of a radial hydrodynamic gas bearing that has radial hydrodynamic grooves in an outer circumferential surface thereof and a disk  3  of a thrust hydrodynamic gas bearing that has thrust hydrodynamic grooves in an upper face thereof are disposed on an upper end of an axial center of a stator core  2  having a stator  2   a  around which a motor coil  7  is wound, a hollow cylinder  6  whose inner surface facing the cylinder  4  of the radial hydrodynamic gas bearing is smooth and a rotor magnet  8  facing the motor coil  7  are disposed on a hub  5  acting as a rotational member, a load in a radial direction is supported by the radial hydrodynamic gas bearing, and a load in a thrust direction is supported by using the thrust hydrodynamic gas bearing together with a magnetic bearing consisting of the stator  2   a  and the rotor magnet  8.

TECHNICAL FIELD

The present invention relates to, in a rotating device in which a spindle motor provided with a hydrodynamic gas bearing that has a radial hydrodynamic gas bearing and a thrust hydrodynamic gas bearing acts as a driving source for a rotator, such as a magnetic disk, an optical disk, or a polygon mirror, a structure for preventing the wear of a thrust hydrodynamic gas bearing surface caused by contacting/sliding especially when it starts.

BACKGROUND ART

In a rotating device for a rotator, such as a magnetic disk, an optical disk, or a polygon mirror, it is widely known that a spindle motor provided with a hydrodynamic gas bearing is employed as a driving source for the rotator. The reason is that the motor is characterized in, for example, that the hydrodynamic gas bearing is simple in structure and can be made more compact, that its noncontact rotation during steady-state rotation does not generate vibrations or rotational irregularity that is caused by the bearing, that it is superior in high-speed durability, and that there is no contamination caused by the dispersion of a lubricant because oil, grease, or the like, is not used.

However, the disadvantage of the spindle motor provided with the hydrodynamic gas bearing is that the hydrodynamic surface of the thrust bearing is in contact when stopped, which causes wear of the surface thereof due to contacting/sliding at the onset of operation. In order to overcome this disadvantage, there is a means in which the hydrodynamic surface of the thrust bearing is floated when stopped, a thrust load is then received by an axial center part of a cylinder of the radial hydrodynamic gas bearing that is a fixed member, and the axial center part of the cylinder is spaced by the thrust of radial hydrodynamic grooves generated with the increase of the revolution speed of the spindle motor so as to maintain the gap of the thrust bearing to have a set value.

Its embodiment is proposed in Japanese Unexamined Patent Publication No. 69715 of 1999. FIG. 10 shows the structure of a shaft fixing type spindle motor 100 therein. 101 is a base plate of a stator 110, 102 is a cylindrical member used also as a shaft erected on the base plate 101, and 103 is a hollow cylindrical member that has a closed end. The hollow cylindrical member 103, the closed end of which is placed upward, is rotatably fitted onto the cylindrical member 102. A donut-shaped thrust member 104 is integrally formed on the outer periphery of the hollow cylindrical member 103, and, at a position opposite to this, a thrust pressure member 106 is disposed through a cover 105 that engages with the base plate 101. A rotor 108 acting as a rotator is fixed to a hub 107 formed integrally with the hollow cylindrical member 103. A rotor magnet 109 is disposed on the outer circumferential surface of the lower part of the hollow cylindrical member 103, and, at a position opposite to this, a motor coil 111 wound around the stator 110 that extends from the base plate 101 is disposed.

When the spindle motor 100 is stopped, the closed end of the hollow cylindrical member 103 and the top of the cylindrical member 102 come into contact with each other by the weight of the rotor 108 including the hub 107, and a gap between the thrust member 104 and the thrust pressure member 106 is sufficiently secured. When an electric current is passed through the motor coil 111, the hollow cylindrical member 103 rotates clockwise, viewed from the side of the rotor 108. And, as its revolution speed increases, a thrust occurs at a herringbone groove 112 formed largely in the upper part of the outer circumferential surface of the cylindrical member 102, whereby the closed end of the hollow cylindrical member 103 and the top of the cylindrical member 102 draw away from each other. Simultaneously, by thrust hydrodynamic that is generated by a spiral groove (not shown) formed in the upper face of the thrust member 104, a gap to the thrust pressure member 106 is reduced and the rotor 108 floats to a position where the thrust and the thrust hydrodynamic preserve a balance.

The publication states that, by the construction as described above, the thrust member 104 and the thrust pressure member 106 are prevented from contact and sliding during the steady-state rotation, and rises no wear whatsoever occurs in this part. Additionally, it says that the rotor 108 up by the thrust generated in the herringbone groove 112 of the radial hydrodynamic gas bearing, and therefore it is possible to obtain a compact spindle motor in which extra additional means other than the hydrodynamic gas bearing are omitted.

However, in the structure of the spindle motor of FIG. 10, since the thrust member 104 disposed on the outer periphery of the hollow cylindrical member 103 must undergo processing to form a spiral groove, the shape becomes complex, and an integral construction is uneconomical. Additionally, since the thrust pressure member 106 is situated above the thrust member 104 which floats upward, the base plate 101 and the cover 105 are required to undergo processing for centering, thus making the shape complex and the assembly difficult. Furthermore, because of the accumulative errors of these interrelated members, it is extremely difficult to maintain the gap of the thrust hydrodynamic gas bearing to be several-microns in order. Therefore, in order to solve the aforementioned problem, the present invention provides a spindle motor capable of preventing contact between a fixed member and a rotating member also when stopped.

DISCLOSURE OF THE INVENTION

In a first embodiment, a cylinder of a radial hydrodynamic gas bearing that has radial hydrodynamic grooves in an outer circumferential surface thereof and a disk of a thrust hydrodynamic gas bearing that has thrust hydrodynamic grooves in an upper face thereof are disposed on an upper end on an axial center of a stator core having a stator around which a motor coil is wound; a hollow cylinder whose inner surface facing the cylinder of the radial hydrodynamic gas bearing is smooth and a rotor magnet facing the motor coil are disposed on a hub acting as a rotational member; a load in a radial direction is supported by the radial hydrodynamic gas bearing; and a load in a thrust direction is supported by using the thrust hydrodynamic gas bearing together with a magnetic bearing consisting of the stator and the rotor magnet.

In a second embodiment, a hub acting as a rotational member is disposed on an upper end of a motor shaft provided with a rotor magnet on an outer periphery thereof, below the motor shaft, there are disposed a disk of a thrust hydrodynamic gas bearing that has thrust hydrodynamic grooves in a lower face thereof and a cylinder of a radial hydrodynamic gas bearing that has radial hydrodynamic grooves in an outer circumferential surface thereof, as a fixed member, there are disposed a hollow cylinder whose inner surface facing the cylinder of the radial hydrodynamic gas bearing is smooth and a stator around which a motor coil is wound, the stator facing the rotor magnet; a load in a radial direction is supported by the radial hydrodynamic gas bearing; and a load in a thrust direction is supported by using the thrust hydrodynamic gas bearing together with a magnetic bearing consisting of the stator and the rotor magnet.

In a third embodiment, a cylinder of a radial hydrodynamic gas bearing that has radial hydrodynamic grooves in an outer circumferential surface thereof and a disk of a thrust hydrodynamic gas bearing that has thrust hydrodynamic grooves in an upper face thereof are disposed on an upper end on an axial center of a stator core having a stator around which a motor coil is wound; a hollow cylinder whose inner surface facing the cylinder of the radial hydrodynamic gas bearing is smooth and a rotor magnet facing the motor coil are disposed on a hub acting as a rotational member; a secondary magnetic bearing is disposed that comprises a first permanent magnet shaped like a ring, the first permanent magnet fixed to an upper end surface of the cylinder, and a second permanent magnet shaped like a ring, the second permanent magnet fixed to an upper end surface of the hollow cylinder in such a way as to surround the first permanent magnet; a load in a radial direction is supported by the radial hydrodynamic gas bearing; and a load in a thrust direction is supported by using together the thrust hydrodynamic gas bearing, the secondary magnetic bearing, and a primary magnetic bearing consisting of the stator and the rotor magnet.

In a fourth embodiment, a hub acting as a rotational member is disposed on an upper end of a motor shaft provided with a rotor magnet on an outer periphery thereof; below the motor shaft, there are disposed a disk of a thrust hydrodynamic gas bearing that has thrust hydrodynamic grooves in a lower face thereof and a cylinder of a radial hydrodynamic gas bearing that has radial hydrodynamic grooves in an outer circumferential surface thereof; in a case as a fixed member, there are disposed a hollow cylinder whose inner surface facing the cylinder of the radial hydrodynamic gas bearing is smooth and a stator around which a motor coil is wound, the stator facing the rotor magnet; a secondary magnetic bearing that comprises a first permanent magnet shaped like a ring is disposed, the first permanent magnet fixed to a lower end surface of the cylinder, and a second permanent magnet shaped like a ring, the second permanent magnet fixed to a lower end surface of the hollow cylinder in such a way as to surround the first permanent magnet; a load in a radial direction is supported by the radial hydrodynamic gas bearing; and a load in a thrust direction is supported by using together the thrust hydrodynamic gas bearing, the secondary magnetic bearing, and a primary magnetic bearing consisting of the stator and the rotor magnet.

Preferably, the radial hydrodynamic grooves which exist in each embodiment consist of at least three groove lines, each lead terminal of which is formed in a range so as not to extend beyond the starting point of an adjacent groove line in a development.

Additionally, even if the radial hydrodynamic groove that exists in each embodiment is a herringbone groove having a groove length asymmetrical to the upper and lower parts, a similar effect is obtained.

And, it is preferable to use light, hard silicon nitride ceramics, silicon carbide ceramics, or alumina ceramics for members making up a radial hydrodynamic gas bearing and a thrust hydrodynamic gas bearing

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1 b are sectional views of the first embodiment of the spindle motor of the present invention.

FIGS. 2a and 2 b are sectional views of the second embodiment of the spindle motor of the present invention.

FIGS. 3a and 3 b are sectional views of the third embodiment of the spindle motor of the present invention.

FIGS. 4a and 4 b are sectional views of the fourth embodiment of the spindle motor of the present invention.

FIG. 5 shows an example in which a herringbone groove is formed as thrust hydrodynamic grooves in the first embodiment of the present invention.

FIG. 6 shows an example in which herringbone grooves are formed as thrust hydrodynamic grooves in the third embodiment of the present invention.

FIG. 7 is an outline view of radial hydrodynamic grooves of the present invention.

FIG. 8 is a sectional view of a spindle motor of a first comparative

FIG. 9 is a sectional view of a spindle motor of a second comparative example.

FIG. 10 is a sectional view of a conventional spindle motor.

EMBODIMENT 1

Preferred embodiments in which the present invention is embodied will be described in detail hereinafter with reference to the attached drawings. FIG. 1 shows the structure of a first embodiment in which a magnetic bearing is constructed between a stator and a rotor magnet, wherein (a) is a diagram of a state when stopped, and (b) is a diagram of steady-state rotation.

In FIG. 1(a), reference character 1 designates a base plate for fixing a stator core 2 as a fixed member. A stator 2 a that is made of a silicon steel plate is disposed integrally with the outer periphery of the stator core 2, and is encircled with a motor coil 7. On the axial center of the stator core 2, there are fixed a disk 3 of a thrust hydrodynamic gas bearing that has a spiral groove 3 a acting as thrust hydrodynamic grooves and so forth in its upper face and a cylinder 4 of a radial hydrodynamic gas bearing that has spiral grooves 4 a acting as radial hydrodynamic grooves in its outer circumferential surface. And, a hollow cylinder 6 an inner surface of which is smooth is fixed at a position where it faces the cylinder 4 inside a hub 5 acting as a rotational member. Furthermore, a rotor magnet 8 is disposed in the shape of a ring at a position where it faces a motor coil of a skirt 5 a of the hub 5. It is noted that a gap between the cylinder 4 and the hollow cylinder 6 is set to have a difference of 5 μm or less in diameter. Rotators (not shown), such as a magnetic disk, an optical disk, and a polygon mirror, are mounted on the outer periphery of the hub 5.

The rotor magnet 8 and the stator 2 a constitute a magnetic bearing. When the spindle motor is stopped, the center location (ii) of the rotor magnet 8 should rest at the center location (i) of the stator 2 a that has the strongest magnetic force, but, according to a balance between the weight of the hub 5 and the magnetic force, it rests at a position slightly descending from the center location (i), because no electric current is passed through the motor coil 7. A clearance between the upper surface of the disk 3 that constitutes the thrust hydrodynamic gas bearing at this time and the bottom surface of the hollow cylinder 6 becomes A1, and they are in the state of non-contact.

As shown in FIG. 1(b), when an electric current is passed through the motor coil 7 and thereby an alternating field is generated in the stator 2 a, the hollow cylinder 6 rotates counterclockwise, viewed from the side of the hub 5. As the revolution speed increases, the hub 5 descends because of the hydrodynamic of the hollow cylinder 6 that constitutes the radial hydrodynamic gas bearing and the spiral grooves 4 a in the outer circumferential surface of the cylinder 4, so that a clearance between the upper surface of the disk 3 and the bottom surface of the hollow cylinder 6 becomes A2 as a result of the balance with repulsion that occurs in the spiral grooves 3 a. Normally, the revolution speed of the hub 5 reaches 12000 to 18000 rpm, and A2 is maintained at the clearance of 2 to 3 μm. During steady-state rotation, since the hub 5 descends because of the hydrodynamic of the spiral grooves 4 a, the center location (ii) of the rotor magnet 8 becomes lower than when stopped, and is deviated from (i) by A3, thus slightly decreasing the motor efficiency.

FIG. 2 shows the structure of a second embodiment in which a magnetic bearing is constructed between the stator 2 a and the rotor magnet 8 in the same manner as in the first embodiment, with the side of the hollow cylinder 6 functioning as a fixed member. FIG. 2(a) is a diagram of a state when stopped, and FIG. 2(b) is a diagram of steady-state rotation.

In FIG. 2(a), reference character 9 designates a motor shaft, on the outer periphery of which the rotor magnet 8 is disposed, and a hub 5 acting as a rotational member is fixed to the upper end. Below the motor shaft 9, there are fixed a disk 3 of a thrust hydrodynamic gas bearing that has spiral grooves 3 a, etc., acting as thrust hydrodynamic grooves in its undersurface and a cylinder 4 of a radial hydrodynamic gas bearing that has spiral grooves 4 a acting as radial hydrodynamic grooves in its outer circumferential surface. A hollow cylinder 6 an inner surface of which is smooth is fixed to a case 10 as a fixed member at a position facing the cylinder 4, and a stator 2 a that is made of a silicon steel plate and is encircled with a motor coil 7 is disposed integrally therewith at a position facing the rotor magnet 8.

When stopped, the undersurface of the disk 3 that constitutes the thrust hydrodynamic gas bearing and the upper end surface of the hollow cylinder 6 are mutually in a state of non-contact, and its clearance is A1. When an electric current is passed through the motor coil 7 and thereby an alternating field is generated in the stator 2 a, the cylinder 4 rotates counterclockwise, viewed from the side of the hub 5, as shown in FIG. 2(b), and the hub 5 descends. A clearance between the undersurface of the disk 3 and the upper end surface of the hollow cylinder 6 becomes A2 as a result of the balance with repulsion generated by the spiral grooves 3 a. Accordingly, the same operational effect as in the first embodiment is obtained. The center location (ii) of the rotor magnet 8 moves downward during steady-state rotation, and deviates from the center location (i) of the motor coil 7 by A3, thus decreasing the motor efficiency slightly, in the same manner as in the first embodiment.

FIG. 3 shows the structure of a third embodiment in which a ring-shaped permanent magnet is fixed onto the concentric circle of the respective upper end surfaces of the cylinder 4 and the hollow cylinder 6 so as to construct a secondary magnetic bearing. FIG. 3(a) is a diagram of a state when stopped, and FIG. 3(b) is a diagram of steady-state rotation.

In FIG. 3(a), reference character 1 designates a base plate for fixing a stator core 2 as a fixed member. A stator 2 a that is made of a silicon steel plate is disposed integrally with the outer periphery of the stator core 2, and is encircled with a motor coil 7. On the axial center of the stator core 2, there are fixed a disk 3 of a thrust hydrodynamic gas bearing that has a spiral groove 3 a acting as thrust hydrodynamic grooves and so forth in its upper face and a cylinder 4 of a radial hydrodynamic gas bearing that has spiral grooves 4 a acting as radial hydrodynamic grooves in its outer circumferential surface. And, a hollow cylinder 6 an inner surface of which is smooth is fixed at a position where it faces the cylinder 4 inside a hub 5 acting as a rotational member. Furthermore, a rotor magnet 8 is disposed in the shape of a ring at a position where it faces a motor coil of a skirt 5 a of the hub 5, thereby constructing a primary magnetic bearing, in exactly the same manner as in the first embodiment.

In the first embodiment, the center location (iv) of the rotor magnet 8 sinks below the center location (iii) of the stator 2 a because of the own weight of the hub 5 when stopped, but, in the third embodiment, in order to prevent this, a first permanent magnet 11 shaped like a ring is fixed to the upper end surface of the cylinder 4, and a second permanent magnet 12 shaped like a ring is fixed to the upper end surface of the hollow cylinder 6 in a manner so as to surround the first permanent magnet 11, thereby constructing a secondary magnetic bearing. If the outside of the first permanent magnet 11 is designed as a south pole and, on the other hand, the inside of the second permanent magnet 12 is designed as a north pole, or vice versa, the respective permanent magnets 11 and 12 attract each other so as to support the weight of the hub 5, and the center location (iv) of the rotor magnet 8 rests at a position higher than the center location (iii) of the stator 2 a. The outer diameter of the first permanent magnet 11 is set at a slightly smaller value than the outer diameter of the cylinder 4, and the inner diameter of the second permanent magnet 12 is set at a slightly larger value than the inner diameter of the hollow cylinder 6, and they are fixed, for example, by an anaerobic adhesive that is hardened by ultraviolet rays. Accordingly, by enlarging a face-to-face gap between the permanent magnets 11 and 12 to be more than a face-to-face gap of the radial hydrodynamic gas bearing, the cylinder 4 and the hollow cylinder 6 are brought into contact together before the contact between the permanent magnets 11 and 12, thus facilitating the heating processing of the adhesive also.

The clearance between the upper surface of the disk 3 and the bottom surface of the hollow cylinder 6 when stopped in FIG. 3(a) becomes A1, and they are mutually maintained in the state of non-contact. When an electric current is passed through the motor coil 7, and the hollow cylinder 6 reaches the state of steady-state rotation in which the hollow cylinder 6 rotates counterclockwise viewed from the side of the hub 5, the hub 5 descends to the position of A2 because of the hydrodynamic in the spiral grooves 4 a in the outer circumferential surface of the cylinder 4 and the hollow cylinder 6 that constitute a radial hydrodynamic gas bearing. In this state, since the center (iv) of the rotor magnet 8 coincides with the center (iii) of the motor coil 7 in the optimum zone of magnetic lines of force, the efficiency of the motor can be optimized.

FIG. 4 shows the structure of a fourth embodiment in which a secondary magnetic bearing is constructed in the same manner as in the third embodiment, with the side of the hollow cylinder 6 functioning as a fixed member. FIG. 4(a) is a diagram of a state when stopped, and FIG. 4(b) is a diagram of steady-state rotation.

In FIG. 4(a), reference character 9 designates a motor shaft, on the outer periphery of which the rotor magnet 8 is disposed, and a hub 5 acting as a rotational member is fixed to the upper end. Below the motor shaft 9, there are fixed a disk 3 of a thrust hydrodynamic gas bearing that has spiral grooves 3 a, etc., acting as thrust hydrodynamic grooves in its undersurface and a cylinder 4 of a radial hydrodynamic gas bearing that has a spiral groove 4 a acting as radial hydrodynamic grooves in its outer circumferential surface. A hollow cylinder 6 an inner surface of which is smooth is fixed to a case 10 as a fixed member at a position facing the cylinder 4, and a stator 2 a that is made of a silicon steel plate and is encircled with a motor coil 7 is disposed integrally therewith at a position facing the rotor magnet 8, thereby constructing a primary magnetic bearing, in exactly the same manner as in the second embodiment.

Further, a first permanent magnet 11 shaped like a ring is fixed to the bottom surface of the cylinder 4, and a second permanent magnet 12 shaped like a ring is fixed to the bottom surface of the hollow cylinder 6 in such a way as to surround the first permanent magnet 11 so as to construct a secondary magnetic bearing, in exactly the same manner as in the third embodiment.

The clearance between the disk 3 and the hollow cylinder 6, which are in a non-contact state, when stopped in FIG. 4(a) becomes A1, and when an electric current is passed through the motor coil 7, and the hollow cylinder 6 reaches the state of steady-state rotation in which the hollow cylinder 6 rotates counterclockwise viewed from the side of the hub 5, the hub 5 descends to the position of A2 as shown in FIG. 4(b). In this state, since the center (iv) of the rotor magnet 8 coincides with the center (iii) of the motor coil 7 in the optimum zone of magnetic lines of force, the efficiency of the motor can be optimized as in the third embodiment.

In any of the first to fourth embodiments, the radial hydrodynamic grooves formed in the outer periphery of the cylinder 4 may be herringbone grooves asymmetrical about the upper and lower parts as shown in FIGS. 5 and 6. Additionally, grooves as shown in FIG. 7 is formed, i.e., at least three groove lines 4 a are formed in the circumferential surface. And, it is preferable to form the terminal Y1 of the groove line 4 a in a range so as not to extend beyond the adjacent starting point X2 of the groove line 4 a in a developed state. Additionally, from the starting point X1 to the terminal Y1, the groove line 4 a may be a straight line or a spiral curve. The groove line 4 a is normally 1 to 3 mm in width, and it may be a shallow groove of about several micrometers to several tens of micrometers in depth, and, according to how a whetstone is applied, the hollow cylinder may be formed to have a shaved flat surface. In the aforementioned groove line 4 a, the number of processing steps can be reduced more significantly than with the herringbone groove, and an equal effect is achieved.

In any of the first to fourth embodiments, it is preferable to make at least the disk 3, the cylinder 4, and the hollow cylinder 6 by ceramic-made members, such as alumina, silicon nitride, and silicon carbide. Among these materials, alumina is the most cost-effective.

Next, a description will be provided of the results of preparing and comparing comparative examples 1 and 2 in order to confirm the operational effect of embodiments 1 and 3. FIG. 8 shows the structure during steady-state rotation of a spindle motor of comparative example 1 compared with embodiment 1. The names, signs, and operations of constituent elements in FIG. 8 are exactly the same as in embodiment 1, and therefore their description is omitted. However, there is a difference in that, when the motor is stopped, the upper surface of the disk 3 and the bottom surface of the hollow cylinder 6, which constitute a thrust hydrodynamic gas bearing, are brought into contact together.

FIG. 9 shows the structure during steady-state rotation of a spindle motor of comparative example 2 compared with embodiment 3. The names, signs, and operations of constituent elements in FIG. 9 are exactly the same as in embodiment 3, and therefore their description is omitted. However, the groove line 4 a formed in the outer periphery of the cylinder 4 may be a herringbone groove that is even about the upper and lower parts or may be a longitudinal groove that is parallel to an axial center and straight. And, there is a difference in that the disk 3 that constitutes a thrust hydrodynamic gas bearing is omitted, and a thrust load is supported only by the primary and secondary magnetic bearings.

Table 1 shows results obtained such that the thrust load of embodiments 1, 3 and comparative examples 1, 2 is set constantly at 150 g, the spindle motor, the specification of which is a dc 12V rating, is driven at a steady-state revolving speed of 18000 rpm, and the up-and-down motion of the upper surface of the hub 5 is observed with a laser displacement gauge.

TABLE 1 Current value Up-and-down motion of hub Sample at start during steady-state rotation embodiment 1 1.2 A 1 μm or less embodiment 3 1.2 A 1 μm or less comparative ex. 1 2.8 A 1 μm or less comparative ex. 2 1.2 A max 30 μm

The results in Table 1 show that the up-and-down motion of the hub 5 during steady-state rotation is 1 μm or less in embodiments 1, 3, and comparative example 1, and the magnetic head of a reader is in a range in which it can follow the surface of the rotator of the magnetic disk. Therefore, the rotator never collide with the magnetic head. However, in comparative example 2, a thrust hydrodynamic gas bearing is omitted, and a thrust load is supported only by a magnetic bearing, and therefore the hub 5 floats unstably, which is unpractical.

In comparative example 1, as a current for starting, approximately 2.3 times that of embodiments 1 and 3 and comparative example 2 is required for generating rotation torque conquering the initial sliding resistance since the upper surface of the disk 3 and the lower end surface of the hollow cylinder 6 contact together in the stop condition. Concerning embodiment 1, since the best zone of magnetic lines of force of the rotor magnet 8 deviates slightly from the center of the motor coil 7 during steady-state rotation, the motor efficiency decreases, and a current value has a slight tendency to increase, but it is in a practically negligible range.

INDUSTRIAL APPLICABILITY

When the motor is stopped, the thrust load of a hub is supported by a primary magnetic bearing made up of a stator and a rotor magnet or in combination with a cylinder and a secondary magnetic bearing made up of a pair of ring-shaped permanent magnets disposed on the end face of a hollow cylinder, and the disk of the thrust hydrodynamic gas bearing and the end face of the hollow cylinder are maintained in a non-contact state. When the rotor magnet is activated, a force reducing the clearance of the thrust hydrodynamic gas bearing is generated by the radial hydrodynamic grooves of the radial hydrodynamic gas bearing so that the disk of the thrust hydrodynamic gas bearing and the hollow cylinder approach each other, and, during steady-state rotation, since the thrust load is supported chiefly by the repulsion of the thrust hydrodynamic gas bearing, it is possible to provide a spindle motor in which the constituent elements of the thrust hydrodynamic gas bearing are consistently prevented from coming in to contact together, and there is no need for concern over the wear of the elements. Also less electric power consumption is used at the start time. And, as is clear from FIGS. 1 to 4, since it has a structure that facilitates the centering of the interrelated members, it is also easy to maintain the gap of the thrust hydrodynamic gas bearing to have a value of several micrometers. 

What is claimed is:
 1. A spindle motor characterized in that it comprises a cylinder of a radial hydrodynamic gas bearing that has radial hydrodynamic grooves in an outer circumferential surface thereof, a disk of a thrust hydrodynamic gas bearing that has thrust hydrodynamic grooves in an upper face thereof, the cylinder and the disk being disposed on an upper end on an axial center of a stator core having a stator around which a motor coil is wound, a hollow cylinder whose inner surface facing the cylinder of the radial hydrodynamic gas bearing is smooth, and a rotor magnet facing the motor coil, the hollow cylinder and the rotor magnet being disposed on a hub acting as a rotational member, wherein is disposed a secondary magnetic bearing is disposed that comprises a first permanent magnet shaped like a ring, the first permanent magnet fixed to an upper end surface of the cylinder, and a second permanent magnet shaped like a ring, the second permanent magnet fixed to an upper end surface of the hollow cylinder in such a way as to surround the first permanent magnet, a load in a radial direction is supported by the radial hydrodynamic gas bearing, and a load in a thrust direction is supported by using together the thrust hydrodynamic gas bearing, the secondary magnetic bearing, and a primary magnetic bearing consisting of the stator and the rotor magnet.
 2. A spindle motor characterized in that it comprises a hub acting as a rotational member disposed on an upper end of a motor shaft provided with a rotor magnet on an outer periphery thereof, a disk of a thrust hydrodynamic gas bearing that has thrust hydrodynamic grooves in a lower face thereof, a cylinder of a radial hydrodynamic gas bearing that has radial hydrodynamic grooves in an outer circumferential surface thereof, the disk and the cylinder being disposed below the motor shaft, a hollow cylinder whose inner surface facing the cylinder of the radial hydrodynamic gas bearing is smooth, and a stator around which a motor coil is wound, the stator facing the rotor magnet, the hollow cylinder and the stator being disposed in a case as a fixed member, wherein a secondary magnetic bearing is disposed that comprises a first permanent magnet shaped like a ring, the first permanent magnet fixed to a lower end surface of the cylinder, and a second permanent magnet shaped like a ring, the second permanent magnet fixed to a lower end surface of the hollow cylinder in such a way as to surround the first permanent magnet, a load in a radial direction is supported by the radial hydrodynamic gas bearing, and a load in a thrust direction is supported by using together the thrust hydrodynamic gas bearing, the secondary magnetic bearing, and a primary magnetic bearing consisting of the stator and the rotor magnet. 